Engine manufacturers have focused on natural gas spark ignition engines due to the strict regulations for the reduction of emission levels. However, the characteristics of injectors play an important role in mixture formation and combustion. Two methods to identify the injection characteristics of the gaseous fuel injector which is applied in Spark Ignition Direct Injection (SIDI) engine are described. The first method studies the structure of Compressed Natural Gas (CNG) jet which is directly injected into an optical constant volume chamber for different pressure ratios. Schlieren photography is used to visualize the jet development. The correlation of tip penetration for near and far field of the jet is presented. The second experiment investigates the variations of mass flow rate due to variation of injection pressure and pressure of chamber. The results of this method indicate that mass flow rate through the injector is affected by the injection pressure linearly while it is not influenced by the pressure of chamber. (C) 2015 Elsevier Ltd. All rights reserved.